Trace Element and Stable Isotope Geochemistry of Lwamondo and Zebediela Kaolins, Limpopo Province, South Africa: Implication for Paleoenvironmental Reconstruction
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minerals Article Trace Element and Stable Isotope Geochemistry of Lwamondo and Zebediela Kaolins, Limpopo Province, South Africa: Implication for Paleoenvironmental Reconstruction Avhatakali Raphalalani 1, Georges-Ivo Ekosse 2,*, John Odiyo 1, Jason Ogola 1 and Nenita Bukalo 1 1 School of Environmental Sciences, University of Venda, Thohoyandou 0950, Limpopo Province, South Africa; [email protected] (A.R.); [email protected] (J.O.); [email protected] (J.O.); [email protected] (N.B.) 2 Directorate of Research and Innovation, University of Venda, Thohoyandou 0950, Limpopo Province, South Africa * Correspondence: [email protected]; Tel.: +27-159-628-504 Received: 6 December 2018; Accepted: 31 January 2019; Published: 4 February 2019 Abstract: The aim of the present study was the paleoenvironmental reconstruction of the prevailing environment under which the Lwamondo and Zebediela kaolin deposits were formed. Hence, this study reports deuterium and oxygen stable isotope values and trace and rare earth element concentrations for two samples of kaolin. Upper continental crust-normalised trace-element patterns reveal that large ion lithophile elements and high-field-strength elements are generally depleted in Lwamondo and Zebediela kaolins, whereas transition trace elements are generally enriched in these kaolins. Upper continental crust-normalised rare earth element (REE) patterns show that there is a slight enrichment of heavy REEs (HREEs) compared to light REEs (LREEs) in these kaolins. The d18O and dD stable isotope values for kaolinite from Lwamondo ranged from 17.4 to 19.1 and from −54 to 84 , respectively, whereas those values for kaolinite from Zebedielah varied fromh 15.6 to 17.7h and fromh −61 to −68 for d18O and dD, respectively. The REE patterns and the contenth of otherh trace elementsh indicateh ongoing kaolinitisation in the Lwamondo and Zebediela kaolins with minimum mineral sorting. The sources of the kaolins varied from basic to acidic and these were derived from an active margin tectonic setting. Lwamondo kaolin was deposited in an oxic environment whereas Zebediela kaolin was deposited under suboxic/anoxic conditions. Based on the d18O and dD values of the kaolinite, they formed in a supergene environment at temperatures generally below 40 ◦C. Keywords: kaolinitisation; mineral sorting; rare earth elements; source rocks; supergene environment 1. Introduction Kaolin refers to a clayey rock and also to a group of clay minerals consisting of kaolinite, dickite, nacrite and halloysite. Kaolinite is the most abundant kaolin mineral, whereas dickite, nacrite and halloysite are relatively rare, being commonly formed by hydrothermal alteration and occurring in sedimentary and residual deposits in association with kaolinite [1]. Kaolin is used in many industrial applications, including paper filling and coating, refractory, ceramics, fibreglass, cement, rubber, plastics, paint, catalyst and many other uses [2]. Considering its wide usage, any occurrence of kaolin is worth proper chemical, mineralogical and technological investigations [3]. A better understanding Minerals 2019, 9, 93; doi:10.3390/min9020093 www.mdpi.com/journal/minerals Minerals 2019, 9, 93 2 of 19 of kaolins hinge on their environments of formation. Only a few studies have been carried out on the utilisation of kaolin in paleoenvironmental reconstruction. Trace element and stable isotope compositions of kaolins are powerful geochemical proxies in reconstructing paleoenvironments. Trace elements (rare earth elements (REEs), large ion lithophile elements, high-field-strength elements and transition trace elements) are less mobile during sedimentary processes. This low mobility enables them to stay in the solid phase during erosion and sedimentation, thereby preserving the chemical signatures of their source rocks [4]. The concentrations of trace elements and specific ratios provide information on paleoweathering, sedimentary provenance, paleotectonics, hydraulic sorting, sediment recycling and paleo-redox conditions prevailing during the formation of kaolins [4–10]. The stable isotopic composition of kaolins can provide paleoenvironmental information on the isotopic composition of soil and meteoric waters, as well as paleotemperatures during the formation of the kaolins [11]. The stable isotope geochemistry of clay minerals has been applied in geothermometry, clay mineral genesis and diagenesis and paleoenvironments, amongst other disciplines [12–17]. In South Africa, there are a number of kaolin deposits distributed throughout the country. Most of the deposits have been studied for their geological, mineralogical and petrological properties. In Limpopo Province, the deposits have not been fully characterised, though some are being exploited for the making of bricks. This study, which is part of a mega-research on clays and clay minerals in Africa, focused on utilising trace elements and stable isotopes in two kaolins from the Lwamondo and Zebediela deposits in Limpopo Province, South Africa, to reconstruct the paleoenvironments in which they were formed. 2. Materials and Methods 2.1. Geologic Setting The Lwamondo kaolin covers an area of about 2700 m2 with an average thickness of 28 m. Half of the deposit has already been mined [18]. The Zebediela kaolin outcrop covers an area of about 3 km2, having an average thickness of 130 m and an estimated volume of 390,000,000 m3. Open pit mining is on-going, with only 40% of the deposit having been mined [18]. The Lwamondo and Zebediela kaolins are classified as secondary kaolins. The geology of the Lwamondo area is dominated by the Soutpansberg Group which is Proterozoic in age (Figure1). The rocks of the Soutpansberg Group overlie the eastern part of the Palala shear zone and part of the Kaapvaal Craton [19]. The Soutpansberg Group represents a volcano-sedimentary succession and is subdivided into seven formations: the Tshifhefhe, Sibasa, Funduzi, Willie’s Poort, Nzhelele, Stayt and Mabiligwe formations. Only four of the seven formations occur within the vicinity of the Lwamondo kaolin, namely the Tshifhefhe, Sibasa, Fundudzi and Willie’s Poort formations [18]. The basal discontinuous Tshifhefhe formation is a few meters thick and made up mainly of strongly epidotised clastic sediments which include shale, greywacke and conglomerate. The Sibasa formation is mainly a volcanic succession with rare discontinuous intercalations of clastic sediments, having a maximum thickness of about 3000 m [19]. The volcanics of the Sibasa formation comprise basalts, which were subaerially extruded and minor pyroclastic rocks [20]. The basalts are amygdaloidal, massive and generally epidotised. The clastic sediments, which include quartzite, shale and minor conglomerate, can locally have a maximum thickness of 400 m [19]. The overlying Fundudzi formation is only developed in the eastern part of the Soutpansberg Group and wedges out towards the west. Minerals 2019, 9, 93 3 of 19 Minerals 2019, 9, 93 3 of 20 Figure 1. Figure 1.Geological Geological setting setting of of the theLwamondo Lwamondo kaolinkaolin occurrence. Modified Modified from from [18]. [18 ]. TheThe Lwamondo Lwamondo kaolinkaolin was deposited deposited within within the theSibasa Sibasa formation formation volcanic volcanic succession. succession. The Thelithology lithology of ofthe the country country rocks rocks were were basalt, basalt, clastic clastic sediments sediments which which include include shale shale and and minor minor conglomeratesconglomerates and and minor minor pyroclastic pyroclastic rocks rocks with with a varicoloureda varicoloured kaolin kaolin (white, (white, brownish, brownish, red, red, pink pink and yellow).and yellow). The exposed The exposed outcrop outcrop at Lwamondo at Lwamondo had an had overburden an overburden with anwith average an average thickness thickness of 2–3 of m and2–3 consists m and ofconsists reddish of brownreddish soil. brown Kaolin soil. wasKaolin exposed was exposed in the lowerin the lower portion portion of the of outcrop. the outcrop. TheThe geology geology of of Zebediela Zebediela is is regionally regionally mademade up of rocks rocks that that belong belong to to the the Transvaal Transvaal Supergroup Supergroup (Figure(Figure2). 2). The The Transvaal Transvaal Supergroup Supergroup is is preserved preserved in in three three separateseparate basins—thebasins—the Transvaal, Transvaal, Kanye Kanye (Botswana)(Botswana) and and Griqualand Griqualand West West basins basins [21 ]—and[21]—and is subdivided is subdivided into into four four main main lithostratigraphic lithostratigraphic units, namelyunits, thenamely Protobasinal the Protobasinal rocks, Blackrocks, ReefBlack formation, Reef formation, Chuniespoort Chuniespoort Group Group and and Pretoria Pretoria Group Group [22 ]. The[22]. area The is coveredarea is covered by surficial by surficial sediments sediments of the of Karoo the Karoo Supergroup Supergroup and and in thein the northern northern part part of of the Transvaalthe Transvaal Supergroup Supergroup is controlled is controlled by ENE- by and ENE- NNW-trending and NNW-trending faults, which faults, are which common are throughoutcommon thethroughout Kaapvaal Cratonthe Kaapvaal [18]. In Craton the Zebediela [18]. In the area, Zebediela the rocks area, belong the rocks to the belong Wolkberg to the GroupWolkberg followed Group by anfollowed unconformity-bound by an unconformity-bound Black Reef formation Black Reef (BRF), formation which (BRF), in turn which was